Much effort has been expended to combat the deposition of scale, commonly referred to as "boiler scale", from water onto the inner surfaces of piping and equipment through which the water is flowed. The scale consists primarily of carbonates, phosphates, hydroxides, silicates and sulfates of the alkaline earth metals; particularly calcium and magnesium. The scale is formed due to precipitation which occurs when the ionic product exceeds the solubility product. The problem of scale deposition is intensified at higher temperatures because of the peculiar inverse temperature-solubility characteristics of these alkaline earth metal salts in water. Thus the salts precipitate more readily on the hot surfaces of heat exchangers than on cold, reducing heat transfer rates and deleteriously interfering with circulation of the water.
Until recently, for the most part, the battle to prevent or inhibit the deposition of the alkaline earth metal salts has been abandoned in favor of various mechanical and chemical treatments to remove the scale after it has been deposited. Mechanical scale removal often entails disassembly of equipment and generally subjects that equipment to the risk of mechanical damage by cutting and abrading equipment of various designs. Chemical scale removal cannot be effective without acids which are difficult to handle, and corrosive to the equipment being treated.
Currently inorganic polyphosphates are used extensively to inhibit and prevent scale and deposit formation in aqueous systems, mainly because they are effective in substoichiometric or threshold quantities, and they are relatively inexpensive. By "polyphosphates" we mean phosphates having a molar ratio of metal oxide: P.sub.2 O.sub.5 in the range from about 1:1 to about 2:1. Their tendency to hydrolyze somewhat unpredictably has initiated their substitution by phosphonic acids and other polyelectrolytes which are stable in aqueous solutions.
When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale forming cation, it is said to be present in "threshold" amounts. See for example, Hatch and Rice, "Industrial Engineering Chemistry", Vol. 31, pg 51 at 53 (January 1939); Reitemeier and Buehrer, "Journal of Physical Chemistry", Vol. 44, No. 5, pg 535 at 536 (May 1940); Fink and Richardson U.S. Pat. No. 2,358,222; and Hatch U.S. Pat. No. 2,539,305.
Threshold inhibition generally takes place under conditions where a few (that is, 1 to about 10 ppm) of polymeric inhibitor will stabilize in solution from about 100 to several thousand ppm of scale-forming mineral. This is distinguished from sequestration because threshold inhibition occurs at substoichiometric ratios of inhibitor to scale-forming cation, whereas sequestration requires a stoichiometric ratio of sequestrant to scale-forming cation to maintain that cation in solution.
It is also known that stoichiometric and substoichiometric quantities of polymers of acrylic acid and methacrylic acid (hereinafter together referred to as "(meth)acrylic acid", and (M)AA for brevity), and (meth)acrylamide, and derivatives of the foregoing, inhibit scale formation in aqueous systems. By stoichiometric amount, we refer to an amount which is sufficient to cause complete complexing with the compound casing scale in water. Hereinafter the term "sequestering" is used to connote use of a composition in stoichiometric amounts, and "threshold" is used to connote use of substoichiometric amounts.
Such polymers are disclosed in U.S. Pat. Nos. 2,783,200; 2,980,610; 3,285,886; 3,463,730; 3,514,476; 3,518,204; 3,663,448; 3,709,815; 3,709,610; 3,880,765; 3,928,196; 4,029,577; 4,209,398; 4,324,684; and 4,326,980 inter alia, the relevant disclosures of which are included by reference as if fully set forth herein.
Polymers of (M)AA and lower alkenyl carboxylates ("LAC") such as vinyl acetate ("VOAc"); and of (M)AA and lower alkenyl alcohols ("LAOH") such as vinyl alcohol have been made with a wide range of molar ratios for use as binders for molding and core sands, paper coatings, and pigments; as, dispersing agents; and, as a warp size for textile manufacture, inter alia. Most copolymers of (M)AA and LAC are essentially insoluble in water as they contain a minor molar amount of (M)AA, that is, less than 50 moles of (M)AA in 100 moles of monomer mixture forming the copolymer, the remaining major amount being the sum of the other constituent(s) of the copolymer.
As far as we are aware, there was no reason to assume that a LAC or a LAOH might be an effective constituent of a polymer in which there is at least 50 moles of (M)AA for 100 moles of monomer mixture forming the copolymer, whether the copolymer is a bipolymer of (M)AA and LAC (or LAOH), or a terpolymer of (M)AA, LAC (or LAOH) and a lower alkenyl sulfonate such as a salt of a lower alkenyl sulfonate ("SLAS"), such as sodium vinyl sulfonate ("SVS"). The term "bipolymer" is used herein to refer specifically to a copolymer of two monomers, namely either (M)AA and LAC or (M)AA and LAOH, or mixtures of these bipolymers.
It is to be noted that U.S. Pat. No. 3,928,196 teaches a copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid which copolymer is not a terpolymer. Further, lower N-alkylacrylamido sulfonates are quite different from lower alkenyl sulfonates.
It is particularly surprising that our copolymers are effective for inhibiting scale deposition, for example, of calcium phosphate, even after the copolymers are hydrolyzed so as to form a copolymer in which one of the repeating units is an alcohol and another repeating unit is a carboxylic acid; the more so, because the effectiveness of known calcium phosphate inhibitors is greatly reduced after being subjected to hydrolytic conditions which convert the unhydrolyzed copolymer to hydrolyzed copolymer which is substantially polyacylic acid or its salts which have low activity as calcium phosphate inhibitors. Thus there was no reason to assume the products of hydrolysis of our copolymers would be effective to inhibit scale deposition.
Deposits of solid particulate matter also occur from an aqueous medium such as industrial water in which is suspended clay and metal oxides, particularly iron oxide (collectively referred to as "mud" or "sludge"). Such deposits may occur in conjunction with deposits resulting from precipitation of salts ("scale") from the water, or even when no scale is formed. To keep the mud in suspension, industrial water is treated with one or more dispersants or sludge-conditioning agents. Choosing a dispersant which is compatible with a scale inhibitor is not an easy task. It is therefore especially noteworthy that the water treatment disclosed herein is effective not only to inhibit the deposition of scale, but also to disperse mud and keep it in suspension.